Escherichia coli is the most common cause of community-acquired urinary tract infection (UTI) and a leading cause of nosocomial UTIs and sepsis. During the past grant period we focused on a genetic island at argW in E.coli urosepsis strain CFT073 which contains the dsdCXA genes for Dserine utilization and ipuAB, homologs of fimBE, the type 1 fimbriae phase-switch recombinases. A CFT073 dsdA mutant lacking D-serine deaminase is 300-fold more competitive than wild type CFT073 in colonizing the bladder or kidney of experimentally infected mice. Compared to CFT073, 44 and 41 genes were respectively up- and down-regulated in the CFT073 dsdA mutant during murine UTI. Up-regulated genes included the D-serine-responsive D-serine transporter dsdX, as well as genes for P and F1C fimbriae, hemolysin, OmpF, a dipeptide transporter DppA, and several genes with unknown functions. CFT073 as well as other uropathogenic E. coli show (+) chemotaxis toward D-serine, but not L-serine as is the case E. coli K-12. ipuA has fimB-like ON-to-OFF and OFF-to-ON fimS switching activity during murine UTI. CFT073 mutants with only single ipuA, fimB or fimE chromosomal genes present can switch from nonmotile fimS ON to a motile OFF phenotype. ipuA also encodes a fimS-independent reversible phase switch of hemolysin expression. A CFT073 dsdA::lacZ transcriptional fusion undergoes reversible phaseswitching albeit by an unidentified mechanism affected by ipuA and ipuB mutations. Therefore, we generated strong support of our original hypotheses that D-serine represents an important signal for regulation of CFT073 virulence genes and growth in the urinary tract and that the dsdCXAlinked recombinases mediate phase-state regulation of important urovirulence factors besides the type 1 fimbriae. We will investigate the mechanism whereby elevated intracellular D-serine leads to up-regulation of urovirulence genes, characterize loci aside from fimS that we observe are controlled by the recombinases and use the CFT073 dsdA mutant to study the roles P fimbriae and hemolysin in colonization of the murine bladder and kidney. The objective of our project is to identify and characterize virulence genes for E. coli involved in serious human diseases. This work will aid in the development of new chemotherapeutic drugs and vaccine strategies.